Method for assigning priority to traffic between local area networks interconnected via a backbone network

ABSTRACT

A method for assigning priority to traffic between LANs interconnected via a backbone network and connected to the backbone network over a transmission line. To be able to control the maximum delays, especially with traffic having a high interactivity requirement, (a) the length of a frame received from the LAN is determined with a predetermined accuracy, thus obtaining at least two length categories having a respective priority status; (b) at least one address of a pair of addresses formed by the source and destination address of the received frame is compared with the corresponding addresses of frames currently waiting for transmission in a queue of frames with a lower priority status than the priority status corresponding to the length of the received frame, or frames which have been in the queue during a predetermined preceding period of time (Tp); (c) frames to be forwarded are applied to at least two different transmission queues with different priority status on the basis of the classification into length categories and the address checking; (d) at least some of the frames to be transmitted are marked so as to distinguish the priority frames from the other frames; and (e) the marking is checked at a buffering point, and the frames are forwarded in accordance with the marked priority status.

BACKGROUND OF THE INVENTION

The invention relates to a method for assigning priorities to trafficbetween local area networks interconnected via a backbone network.

A local area network (LAN) is a local data network taking care oftraffic between workstations connected to it, such as PCs, and variousdevices providing services to the workstations. A basic LANconfiguration comprises a physical transmission path, i.e. cabling, anetwork server, workstations connected to the cabling by means ofadapter cards, and a network software. A LAN is typically located in asingle building or in several buildings located close to each other,e.g. at one office of a specific organization or company. Recently,however, there has been an increased demand to interconnect individualLANs into larger networks. For such purposes, the above-described basiccomponents will not suffice.

Equipments required to interconnect LANs are typically classified inaccordance with the OSI (Open Systems Interconnection) model of the ISO(International Standards Organization). The OSI model aims at creating aframework for standards applied in data transmission between opensystems. The model comprises seven superimposed layers the tasks ofwhich have been specified whereas their implementation has been leftopen. The OSI model is described more closely, e.g. in Reference [1](the references are listed at the end of this specification).

The devices used in the interconnection of LANs, i.e. the repeater, thebridge, and the router, will be described briefly hereinbelow.

The repeater is the simplest means used in the interconnection of LANsor LAN segments. The repeater operates on the lowest OSI layer (layer1), i.e. on the physical layer. The repeater amplifies the bit streamand forwards all traffic over it from one network segment to another.The repeater is thus used to increase the physical length of thenetwork, and it can be used only when the networks to be interconnectedare fully identical (or differ only in the transmission medium).Segments interconnected by the repeater form a single logical network.

The bridge operates on the next OSI layer (layer 2), i.e. on the datalink layer. Even though the data link layer is mainly independent of thephysical transmission medium used, some of its functions are dependenton the physical transmission medium. For this reason, the data linklayer includes a so-called MAC (Media Access Control) sublayer in somenetwork architectures. The MAC sublayer provides access to thetransmission path, i.e. it takes care of functions most probablyassociated with the characteristics of the physical transmission path.Bridges typically operate on the MAC sublayer. The function of thebridge is to monitor frames transferred over the LAN and to transferthem from one network to another on the basis of the physical address ofthe data packets. Only frames having a destination address indicatingtransmission to the side of the other network are able to cross thebridge. The bridge thus acts as an insulator which reduces the load inother network portions. The bridge does not analyze more closely whatthe frames transport, and it ignores the higher-level protocoltransferred in the frame. In other words, the bridge isprotocol-independent, and so it can be used to interconnect networksutilizing protocols of different types (TCP/IP, XNS, OSI, NetBios,etc.).

The bridge contains a so-called routing table, which is updated by thebridge on the basis of the addresses of the transmitting parties offrames received by the bridge. The routing table indicates to the bridgebehind which interface a specific station is currently located. In otherwords, the bridge is able to "learn" station locations so that newstations can be added to the network without having to reconfigure thebridge.

The router operates on the third OSI layer, i.e. on the network layer.Routers know the higher protocols used in the LAN traffic and routemessages by means of the addressing mechanisms of these protocols. Therouter forwards the frame (to another router or to a destinationstation) on the basis of the data obtained from the network addressrouting table. The router calculates an optimal route for each frame.The maintenance of routing data and the route selection are based on arouting protocol utilized by the router (such as RIP, RoutingInformation Protocol). The filtering and management properties ofrouters are superior to those of bridges, and they offer betterpossibilities for the construction and use of complicated LANconfigurations.

The bridge and the router are described, e.g. in Reference [2], which isreferred to for a more detailed description.

A packet switched network solution (i.e. a network over which packets ofvarying length are transmitted) requires matching of rates as thesubscriber interfaces usually operate at different rates due to theirdifferent data transmission capacity requirements. There also oftenoccur different rates over data links between network nodes.Rate/matching is usually performed by buffering which should not causean excessive absolute delay in order that the applications utilizing thelink would be satisfied with the data transmission service.

At present, traffic between LANs can be considered to consist of trafficof two types: terminal user traffic and datafile transfer traffic.Terminal traffic has a high interactivity requirement (a delay of 2 to300 ms over the link in both directions) as it provides servicesdirectly to LAN users (who very often are less patient). On thecontrary, the transfer of datafiles mainly consists of interequipmenttransfer traffic in which the required delays are not particularlycritical (the delays may be several seconds).

FIG. 1 shows a public network service in which local area networks 11 ofdifferent offices A-E of one specific company are interconnected via apublic network 12 acting as a backbone network. The public network 12 isany network known per se which is able to forward LAN data packets ofvarying length, such as a FR (Frame Relay) network. The differentoffices typically have different interface rates due to their differentdata transmission requirements, resulting from size differences betweenthe offices. The LAN of each office is adapted to the FR service via abridge 13 located at the office and a transmission line. Thetransmission lines are indicated with the references 14a-14e,respectively. Such interconnection is described in more detail inPyhalammi et al, U.S. patent application Ser. No. 08/416,682, filed Apr.5, 1995, which is referred to for a more detailed description.

The link between the offices A and B will be described by way of examplebelow. The office B transmits first a packet burst to the office A. Asthe office LAN operates at a high rate (e.g. 10 Mbit/s), rate/matchingis required for the subscriber line 14b having a capacity of 64 kbit/sin this example (FIG. 2). The matching is performed by the bridge 13located within the premises of the office B, by buffering the packetsand forwarding them at an access rate to the network 12. Busses withinthe network 12 have a higher capacity than the subscriber lines, and sothere is no need for buffering there. In this case, the rate of thesubscriber line of the office A (2 Mbit/s) is also higher than that ofthe office B, so that the office A is able to receive the packets at therate at which they were transmitted from the office B.

Problems occur when the situation is reversed, i.e. when the rate of thetransmitting interface is higher than that of the receiving interface(e.g. traffic from the head office to a branch office). In such a case,the entire packet burst passes from the subscriber interface A over thenetwork 12 to the edge of the network, and further to the interface ofthe office B, where the packets have to be buffered to wait for accessto the subscriber line. This situation is illustrated in FIG. 2, where abuffering point is indicated by the reference P and a buffer by thereference numeral 21. In practice, the problem is not the buffering, assuch, but the delay caused by the buffering as the delay disturbs theinteractive work. For instance, if the access rate is 64 kbit/s, thedelay of a single packet of one kilobyte will be 125 ms. If a pluralityof such packets are transmitted concurrently, as usual, the delay willbe intolerable for the interactive operation. This is because if apacket requiring a rapid response is received after this kind of packetburst, it has to wait in a queue for the buffer 21 to be emptied.

SUMMARY OF THE INVENTION

The object of the present invention is to eliminate the above-describeddrawback and to provide a method for assigning priority to trafficrequiring a rapid response over other kind of traffic in theabove-described case. This is achieved by a method according to theinvention which is characterized in that the length of a frame receivedfrom the local area network is determined with a predetermined accuracy,thus obtaining at least two length categories having a respectivepriority status; at least one address of a pair of addresses formed bythe source and destination address of the received frame is comparedwith the corresponding addresses of frames currently waiting fortransmission in a queue of frames having a lower priority status thanthe priority status corresponding to the length of said received frame,or frames which have been in said queue during a predetermined precedingperiod of time; frames to be forwarded are applied into at least twodifferent transmission queues with different priority statuses on thebasis of the classification into the length categories and on the basisof the address checking; at least some of the frames to be transmittedare marked so that the priority frames are distinguishable from theother frames; and said marking is checked at said point requiringbuffering and the frames are forwarded in accordance with the prioritystatus indicated by the marking.

One aspect of the invention is to classify the frames to be transmittedinto different priority categories according to their length and toindicate the priority category of the frame so that frames indicated tohave priority will be forwarded more rapidly at subsequent bufferingpoints. Another aspect of the invention is to ensure that the order offrames transmitted from the same transmitting party to the samereceiving party will not change.

The solution according to the invention allows a considerable reductionto be achieved in delays created over the subscriber lines. In practice,this improvement may readily result in that the delay is reduced toone/tenth of the previous value.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to FIGS. 3 to 6 in the examples shown in the attacheddrawings, in which:

FIG. 1 illustrates a public network service in which the LANs of thedifferent offices of the same company are interconnected via a publicnetwork acting as a backbone network;

FIG. 2 is a more detailed view of buffering carried out in thearrangement shown in FIG. 1;

FIG. 3 is a block diagram illustrating an equipment according to theinvention;

FIG. 3a illustrates a frame used in a public FR network, and theinsertion of a LAN frame into a FR frame;

FIG. 4 is a flow diagram illustrating the priority assignment performedat the transmission end in the method according to the invention;

FIG. 5 is a block diagram illustrating the address checking operationcontained in the priority operation performed at the transmission end;and

FIG. 6 is a block diagram illustrating the frame classificationperformed on the side of the public network prior to the subscriberinterface.

DETAILED DESCRIPTION

FIG. 3 shows the arrangement according to the invention for assigningpriority to traffic. A LAN 11 of an office A is connected to asubscriber line 14a by a bridge 13 assigning priorities to terminaltraffic. The priority/assigning bridge 13 corresponds to a conventionalLAN bridge with the exception that it further comprises means forclassifying frames to be forwarded according to their length, into twocategories; means for analyzing the source and destination addresses ofthe frame to be bridged; two buffers for buffering frames on the basisof the length classification and address/checking, into a queue ofpriority frames and a queue of non-priority frames; and means formarking the frames so that the priority frames are distinguishable fromthe other frames.

The limit value used in the frame/length classification may be, e.g.,200 bytes. This means that frames having a length of or below 200 bytesprobably represent (interactive) terminal traffic, while those having alength above 200 bytes probably represent (less interactive) datafiletransfer traffic. A frame from the LAN, which is to be bridged, isreceived by the bridge 13 in its length measuring unit 31, where theframe/length classification takes place by reading the length of theframe from the frame length field (e.g. from the LENGTH field in anEthernet frame).

In order to ensure that the mutual order of packets transmitted from thesame source address to the same destination address will not change, anaddress/checking unit 32 is added after the length/measuring unit. Theaddress/checking unit has two outputs: a buffer 33a for priority framesis connected to one output and a buffer 33b for non-priority frames isconnected to the other output. Priority over the other frames isassigned to a frame classified as short if there is not or has not beenfor a predetermined period of time Tp a frame which has no priority andwhich is to be transmitted or has been transmitted from the sametransmitting party to the same receiving party as the frame classifiedas short. The buffers are connected to a switching means 34, whichforwards the frames to the output of the bridge (the transmission line14a), so that the priority frames in the buffer 33a are processed priorto the non-priority frames in the buffer 33b.

For the sake of clarity, only portions essential to the invention areshown in FIG. 3. In other respects, the bridge 13 corresponds to theprior art technique. Bridges are typically classified on the basis ofthe routing algorithms they utilize. An essential feature of the bridgeis, however, that it comprises two LAN interfaces and means foranalyzing the physical address of the frame (the OSI layer 2). It isfurther to be noted that the method according to the invention isapplied only to LAN frames which the bridge has authenticated and thedestination address of which refers to a network or network portion onthe other side of the bridge. In addition, it is to be noted that aframe received from the LAN (such as an Ethernet frame) is inserted intothe information field of the FR frame in the bridge (with the exceptionof timing bits and other similar bits) prior to the buffering of theframes, and the completed FR frames are buffered into the buffers 33aand 33b. FIG. 3a shows, first, the insertion of a LAN frame 38 into theinformation field of an FR frame 39, and, second, a typical frameconfiguration of the FR network, in which the address field precedingthe information field consists of two octets (bits 1 to 8), the bit No 2of the first octet being a C/R bit (0 or 1). The logical address (DLCI)comprises the bits 3 to 8 of the first octet and the bits 5 to 8 of thesecond octet. The significance of the C/R bit and the logical address inthe method according to the invention will be described more closelybelow. The FR network and its frame configuration are described in moredetail in Reference [3], which is referred to for a more detaileddescription.

A bridge of the same type is disclosed in Pyhalammi et al, U.S. patentapplication Ser. No. 08/416,682, filed Apr. 5, 1995, which describesmore closely, e.g., the typical LAN traffic profile, on the basis ofwhich the limit value used in the frame length classification isselected.

FIG. 4 is a flow diagram illustrating the assignment of priorities bythe bridge 13. In step 41, a frame to be bridged is received in thelength measuring unit 31, which reads the length from the respectivefield of the frame (step 42). The obtained length is compared with apredetermined limit value (such as 200 bytes) in step 43. If theobtained length is equal to or smaller than the limit value, the frameis applied to the address/checking unit 32, where the addressescontained in the frame are checked (step 44). If it is found (step 45)that the result of the address/checking is positive, the FR frame isapplied from the address/checking unit to the non-priority frame buffer33b (step 46). If the result of the address/checking is negative, theC/R bit of the FR frame is set to a predetermined value (step 47), andthe frame is applied from the address/checking unit to the priorityframe buffer 33a (step 48). (The C/R (Common/Response) bit, alreadyreferred to in relation to FIG. 3a, is intended to be utilized by anapplication program, whereby it may also be used for the priorityassignment according to the invention.)

On the other hand, if it is found in step 43 that the frame lengthexceeds the predetermined limit value, the frame is applied directlythrough the address/checking unit (without address/checking) to thenon-priority frame buffer 33b. The LAN frame is, however, transformedinto an FR frame used in the public network before it is applied to thebuffer.

The above-described address/checking for short frames will be describedmore closely with reference to the block diagram of FIG. 5, whichillustrates the operation of the address/checking unit 32. At the start(step 51), it is checked whether there are frames queuing in thenon-priority frame buffer 33b, or whether there has been such framesduring a preceding period of time Tp. If neither one of the conditionsis fulfilled, the result of the address/checking is negative, and theC/R bit of the FR frame is set to a predetermined state, and the FRframe is fed to the priority frame buffer 33a, as described above. Ifthere are still frames in the queue of non-priority frames, or there hasbeen frames during the period of time Tp, the address (destinationaddress) of the receiving party of the frame to be bridged is read instep 52, whereafter the address is compared with the correspondingaddress of a frame in the queue of non-priority frames, or a frame whichhas been in the queue during the period of time Tp (step 53). If theaddresses are identical, the preceding frame is picked up (step 54), andthe address of the transmitting party (source address) is also read fromthe frame to be bridged. This address is compared with the correspondingaddress of the address picked up (step 56). If these addresses are alsoidentical, the result of the address/checking is positive, and therespective FR frame is fed into the non-priority frame buffer 33b, asdescribed above. If the comparisons in steps 53 or 56 show that eitherthe source addresses or destination addresses deviate from each other,the result of the address/checking is negative, and the C/R bit of theFR frame is set and the FR frame is fed to the priority frame buffer33a, as described above. One then returns to the start, and theprocessing is continued for the next frame in the queue of non-priorityframes or a frame which has been in the queue during the period of timeTp, if there are any such frames left.

Accordingly, short frames are fed mainly to the priority frame buffer33a. However, if the non-priority frame buffer already contains a framein which the pair of the source and destination address corresponds tothe pair of the source and destination address of the short frame, theshort frame is fed to the non-priority frame buffer. In this way, it canbe ensured that the order of packets sent from the same transmittingparty to the same receiving party will not change at the transmissionend. If there is no such frame in the non-priority frame buffer butthere has, however, been such a frame during the preceding period oftime Tp, the short frame is nevertheless fed to the non-priority framebuffer. By checking the predetermined preceding period of time Tp, it isensured that the frames will not be disordered when they are transferredat the buffering point P from the public network 12 to the transmissionline 14b. The length of the period of time Tp corresponds to the lengthof the buffer 21 (and a buffer 36b).

As shown in FIG. 3, the subscriber interface of the office B comprises achecking unit 35 having one output connected to the priority framebuffer 36a and another output connected to the non-priority frame buffer36b. The buffers are connected to a switching means 37, which forwardsthe frames to the transmission line 14b so that the priority frames inthe buffer 36a will be transmitted prior to the non-priority frames inthe buffer 36b.

FIG. 6 illustrates the assignment of priorities by the checking unit 35at the subscriber interface of the office B at the edge of the publicnetwork (in a network node). In step 61, the checking unit receives anFR frame to be transferred to the subscriber line 14b. The received FRframe is subjected to C/R bit/checking (step 62), which is applied tofind out whether the bit has been set at the transmission end. If thebit has been set, the frame is fed to the priority frame buffer 36a(step 64b). If the bit has not been set, the frame is fed to thenon-priority frame buffer 36b (step 64a).

In the bridge at the reception end (in this case in the bridge of theoffice B), the frame used by the LAN 11 (e.g. the Ethernet frame) isagain derived from the FR frame.

As an alternative to the use of the C/R bit of the FR frame, thepriority and non-priority frames can be distinguished from each other byassigning separate logical channels for them, for instance, a logicalchannel 3 for the priority frames and a logical channel 4 for thenon-priority frames. The logical channel is indicated by the DLCI (DataLink Connection Identifier) bits of the address field of the FR frame,illustrated in FIG. 3a and also described in Reference [3]. At thereception end, the logical channel is processed in the same way as theC/R bit for distinguishing the priority frames from the non-priorityframes.

Even though the invention has been described above with reference to theexamples shown in the attached drawings, it is obvious that theinvention is not restricted to them, but can be modified within thescope of the inventive idea disclosed above and in the attached claims.For instance, the priority procedure of short frames may also be appliedelsewhere than at the edge of the public network. In such cases, themonitoring time Tp at the transmission end has to be dimensioned on thebasis of the maximum delay of the entire network.

As the maximum delay of the network and the delay variation can bebrought under control by the method according to the invention, it ispossible to transmit, e.g., small-scale speech traffic or videoinformation over the network. The speech channel or speech channel group(or correspondingly video information) is packed into sufficiently shortpackets, and so the network assigns priority to this traffic. In such asituation the minimum rate of the subscriber interface should be highenough (e.g. 512 kbit/s) in order that the delay would remain smallenough. The frames may also be classified into more than two prioritycategories, e.g. by using the above-described logical channel as adistinguishing criterion.

REFERENCES

[1] Leena Jaakonmaki: Lahiverkko-opas, Suomen Atk-kustannus Oy, 1991.

[2] Michael Grimshaw: LAN Interconnections Technology,Telecommunications, February 1991.

[3] An Overview of Frame Relay Technology, Datapro Management of DataCommunications, McGraw-Hill Incorporated, April 1991.

I claim:
 1. A method for assigning priority to traffic between localarea networks which are interconnected to one another via a backbonenetwork, over respective transmission lines, wherein data is transmittedfrom one local network to another in a succession of varied-lengthframes, each having a source address and a destination address, and databuffering is required in the backbone network at a point where data isto be transmitted over a respective transmission line which has a lowertransmission capacity than does said backbone network,said methodcomprising: (a) determining at said point the length of each respectiveframe which is about to be received by the backbone network from eachrespective local area network, with a predetermined accuracy as towhether said length is at least as along as, or is shorter than a givenlength, and thus, obtaining frames of at least two differentframe-length categories, each at least tentatively having a respective,different frame-priority status, of which at least one is lower thananother and at least one is higher than another; (b) causing each framehaving a length which is longer than said given length to have a markingidentifying that frame as a lower priority frame and sending that frameinto a respective queue for lower priority frames to be transmittedalong a respective said transmission line into said backbone network fortransmission along another respective said transmission line intoanother said local area network; (c) comparing at least one address of apair of addresses formed by the source and destination address of eachrespective about to be received frame, with the corresponding at leastone address of each frame which remains in, or has been in therespective queue for lower priority frames within a time interval ofgiven length ending at the present, and(i) upon finding correspondencein said at least one address, sending the respective about to bereceived frame into said respective queue for lower priority frames, but(ii) upon finding a lack of correspondence in said at least one address,causing the respective about to be received frame to have a markingidentifying that frame as a higher priority frame, and sending thatframe into a respective queue for higher priority frames to betransmitted along a respective said transmission line into said backbonenetwork for transmission along another respective transmission line intoanother said local area network; and (d) checking on priority ofsuccessive leading frames waiting in said queues and forwarding saidleading frames from respective said queues into said backbone networkbased on such checking, giving precedence to leading frames havinghigher priority over those having lower priority.
 2. The method of claim1, wherein:in practicing step (c), both source and destination addressesbetween each pair are compared, and correspondence is found only whenthe respective two source addresses correspond and the respective twodestination addresses correspond.
 3. The method of claim 1, wherein:saidtime interval of given length corresponds to the maximum length of arespective lowest priority one of said frame buffers.
 4. The method ofclaim 1, wherein:for each said transmission line having said queues oflower and higher priority, there are a total of two such queues,respectively including a non-priority queue and a high-priority queue.5. The method of claim 4, wherein:said backbone network is a frame relaynetwork, and, in practicing step (c)(ii), a C/R bit of a respective FRframe is given a predetermined value, for marking the respective frameas being a higher priority frame.
 6. The method of claim 1, wherein:saidbackbone network is a frame relay network, and, in practicing step(c)(ii), a logical channel address is given a predetermined value, formarking the respective frame as being a higher priority frame.
 7. Themethod of claim 1, wherein:at least some of said frames bearing adestination address in common, contain information which is susceptibleof being output as speech at the respective destination address.
 8. Themethod of claim 1, wherein:at least some of said frames bearing adestination address in common, contain information which is susceptibleof being output as video at the respective destination address.